High pressure metal halide arc discharge lamps have established themselves as valuable lighting sources. Such lamps generally comprise an elongated arc tube enclosed within an outer envelope or jacket commonly provided with a screw base at one end. The arc tube contains an ionizable fill including an inert buffer gas, mercury and metallic halides, and two main electrodes. The electrodes are supported by inleads including molybdenum foil portions extending through press seals at the ends of the tube. The foils assure hermetic seals withstanding thermal expansion of the parts.
In order to facilitate starting of the arc discharge, a starter electrode may be provided in the arc tube adjacent to one of the main electrodes. A discharge can be ignited between the starter electrode and the adjacent main electrode at a much lower applied voltage than is required to ignite an arc between the two main electrodes. Once the discharge is started, the ionized gas decreases the resistance between the two main electrodes and, if enough potential is available, the arc transfers and settles in the gap between the main electrodes. A resistor connected in series with the starter electrode limits the current flowing through it.
Metal halide lamps on the whole require higher voltages for reliable starting and operation than do high pressure mercury vapor lamps of corresponding size or rating. Conventional lead-lag ballasts for high pressure mercury vapor lamps do not deliver sufficient voltage for reliable starting. Therefore, circuits have been developed to increase the voltage output delivered by a conventional lead-lag ballast during starting. U.S. Pat. No. 3,900,761 to Freese, et al., describes one such circuit and U.S. Pat. No. 4,097,777 to Bacharowski describes an improvement on the Freese, et al. circuit. A later improvement, U.S. Pat. No. 4,353,012 to Fallier and Proud, describes a pulse injection starting circuit for high intensity metal halide lamps utilizing a spiral line generator to generate a high amplitude starting pulse.